Fiber reinforced composites have become widely recognized over the last fifty years for their usefulness as load-bearing materials having excellent thermal and impact resistance, high tensile strength, good chemical resistance and insulating properties. The term "composite" broadly applies to any combination of individual materials, usually built up in layers. The materials may include, for example, cementitious compositions, ceramics or synthetic materials such as plastic resins.
Generally, in fiber-reinforced plastic composites, fibers, typically of glass or carbon, are impregnated within a resin matrix to create a strengthened material. The resulting material has physical properties that are superior to the individual characteristics of the fibers or the resins. Thus, although the fibers are fragile in nature and susceptible to handling damage, and the resin may be soft and overly pliable, when the fibers are incorporated into the resinous matrix, the material so formed has improved strength and durability. The glass fibers strengthen and stiffen the matrix for load bearing, while the matrix resin binds the fibers together and spreads the load across them, thereby protecting them from impact and environmental deterioration. By selecting the matrix, fiber and manufacturing process, the composites can be tailored to meet desired performance requirements. For example, filament wound composites are made using continuous fibers that conform to a desired shape. To make these composites, one or more multi-filament glass strands or rovings are passed through a bath of resin, then the resin-coated strand is wound onto a mandrel of the desired shape. The shaped article is then cured to solidify the resin.
A variety of polymer matrix resins have been used to design and fabricate fiber-reinforced composites. Generally, these resins may be classified into two categories: thermosetting and thermoplastic resins. The difference between these resins and their selection for making the composites is based on their chemistry. The choice of either thermosetting or thermoplastic resins affects the processing conditions and the final form of the composite material. Both types of resin are comprised of molecular chains, however thermoplastics are processed at high temperatures and maintain their plasticity, enabling them to be reheated and re-shaped more than once. Common thermoplastic resins include polyalphaolefins, nylon, polycarbonate and polyvinyl chloride (PVC). The molecular chains in thermoset resins cross-link during the resin curing process, which is effected using heat and/or a catalyst, and as a result the resin sets into a rigid state. Examples of these resins include polyesters, vinyl esters, phenolics, polybutadienes, polyurethanes, polyimides and epoxies.
While thermosetting resins are preferred in some filament-wound composites because of their good mechanical, electrical and chemical-resistance properties, their ease of handling and their relatively low cost, some deficiencies have however been discovered to be associated with their use in this type of composite. For example, researchers have identified certain failure modes that relate to infrastructure uses of the composites. K. Liao et al., Environmental Durability of Fiber-Reinforced Composites for Infrastructural Applications, Proceedings of the Fourth ITI Bridge NDE Users Group Conference (1995). These failure modes include moisture absorption which leads to chemical breakdown of the polymer; creep resulting in rupture; physical aging, in which the polymer approaches equilibrium below its glass transition temperature; stress corrosion; weathering and fatigue.
Filament wound composites such as pipes are typically subjected to cyclic periods of intense pressure during their use life. Over time, this repeated exposure to periods of high internal pressure causes fatigue. Fatigue results in fracture, matrix cracking or splitting, or fiber-matrix debonding once the fatigue limit of the composite is exceeded. In manufacturing filament wound composites, then, it is necessary to design a composite that will withstand at least the maximum pressure that the composite will encounter during normal use. Typically in the industry, such composites are designed to withstand at least 5 times the rated maximum use pressure intended for the article being manufactured. Therefore, where the article is, for example, a pipe with a rated use pressure of 3,000 psi (pounds per square inch), the pipe is manufactured and tested to ensure that it can initially withstand exposure to pressures of at least 15,000 psi. To test the product, a length of the pipe may be filled with fluid, then repeatedly pressurized at its rated use pressure until signs of fatigue such as cracks, leakage or bursting are observed.
Efforts have been made to improve the strength of the composites and so improve burst strength retention and resistance to fatigue. For example, the amount and type of the components may be changed. However, while modifying the type and amount of the components can be used to affect the final properties of the composites, traditionally there have been limitations to doing so. Increasing the amount of fiber component will provide more rigidity, but if the proportion of fibers to polymer is too high the composite becomes too brittle. Conversely, when the amount of polymer in relation to the fiber component is high, the polymer may be more easily molded, but the strength properties are decreased.
Additionally, it has long been recognized that fiber-reinforced composites are extremely sensitive to the bonding strength between the fiber and the matrix. R. J. Kerans, The Role of the Fiber-Matrix Interface in Ceramic Composites, Ceram. Bull. 68 (2): 429-442 (1989); H. C. Cao et al., Effect Of Interfaces on the Properties of Fiber-Reinforced Ceramics, J. Am. Ceram. Soc. 73:1691 (1990); A. G. Evans et al., The Role Of Interfaces in Fiber-Reinforced Brittle Matrix Composites, Composites Sci. & Tech. 42:3-24 (1991). This recognition has led to significant efforts to modify the interface between the fiber and polymer, and so improve the product strength. Accordingly, improving the compatibility of the sizing on the surface of coated glass fibers with the matrix resin has often been investigated as a means of improving the physical performance characteristics of the composites. Traditionally in the art, this compatibility has been evaluated by measuring the clarity of the composite, as well as the absence of air bubbles within the matrix. In particular, while certain types of reinforced composites such as foamed composites have been designed to incorporate a volume of air or vapor into the composite matrix to expand the resin and provide some durability, the procedure of incorporating air into filament wound composites was not previously contemplated. Rather, up until the time of the present invention, using a foaming agent or other additive to permit vapor entrainment and volume expansion in wound composites has been considered undesirable.
Moreover, dispersion of the fibers and coating of their surfaces by the matrix resin has a significant impact on the properties of the composites. Consequently, many efforts have been made to improve the compatibility of the fibers and matrix resins, and thereby to improve the dispersion and coating of the fibers. This is particularly important in filament winding operations where the composite is formed by passing a multi-filament glass strand or roving through a bath of resin and then winding the resin-coated strand onto a mandrel to form the composite article upon cure of the resin. In such operations, enhancing the ability of the resin to impregnate the strand and surround the fiber has been thought to impart improved physical properties to the resulting composites. Typically, this has been attempted by development of the improved sizing compositions applied to the fibers, in addition to mechanically spreading the fibers in the strand as they pass through the resin bath. However, despite significant advances in the properties obtained in such products by these developments, a need continues for ways to obtain further improvements in these composites. Such a need is met by the products and processes of the invention described herein.